There are number of fields of human endeavor wherein it is useful, if not necessary, to know precisely the location and orientation of an object within a space. Surgery is one such field in which this information is desirable. Surgical navigation systems are available that enable medical personnel to know, with a high degree of precession, the location and orientation of surgical instrument or implant relative to a surgical site on the patient. Often this information is used in surgical procedures to facilitate the accurate removal and shaping of tissue. In an orthopedic surgical procedure, the information provided by the surgical navigation system ensures that an implant is precisely positioned.
Surgical navigation systems and other position-locating systems use different means to identify the locations and orientations of the objects they track. A number of commercially available surgical navigation systems rely on light tracking to determine the position of the tracked object. Some systems include trackers that are attached to the objects being tracked. Each tracker emits a number of light beams. Often light is emitted in the infrared wavelengths. A static device, referred to as a localizer, has light sensitive-receivers. Based on the locations from which the individual light beams are received at a localizer, a processor, also part of the system, determines both the position and orientation of the tracker. Based on this information, the position and orientation of the device attached to the tracker is inferentially determined.
Often, at the start of a medical procedure, the position of the patient's body tissue is mapped into a memory integral with the processor. Based on these data and the inferential determination of the tracked object, the surgical navigation system presents an image on a display that indicates the position of the tracked object relative to the body tissue. This allows a surgeon to virtually “view” the position of the object that is otherwise be concealed by overlying tissue.
In an orthopedic surgical procedure, a surgical navigation system is also used to measure the range of motion of the body limb(s) subject to the procedure. These measurement data facilitate the fitting of the implant to the patient to increase the likelihood of successful outcome of the procedure.
Light-based surgical navigation systems work reasonably well for providing object location and orientation data in a surgical setting. Nevertheless, there is a drawback associated with these systems. A light-based navigation system requires a line-of-sight between the light emitting components and the light-sensitive localizer. If the line is broken, the ability of the system to provide object position and location data may be interrupted. Thus, medical personnel using such system must make a concerted effort to keep their own body parts as well as other surgical devices from entering into the space wherein such lines-of-sight may be present.
If the breaking of a line-of-sight results in the interruption of the generation of the object position and location data, it may be necessary stop the procedure until the system can again provide the data. Such delays reduce the overall efficiency of the surgeon performing the procedure. Moreover, such delays can increase the overall length of time it takes to perform the procedure. This is counter to an objective of modern surgical practice, to perform the procedure as quickly as possible. Surgeons work to this goal to reduce the amount of time the patient is held under anesthesia and his/her body is exposed and open to infection.
Recently, there have been efforts to employ electromagnetic field-sensing systems as surgical navigation systems. Generally, this type of navigation system includes one or more transmitters that emit electromagnetic energy. There is a sensor assembly with one or more transducers that are sensitive to electromagnetic energy. To provide both position and location information about an object, it is typically necessary to transmit plural fields and monitor the strength of each signal at plural antennae. Some of these transmitters emit electromagnetic energy upon being energized by AC drive signals. Others of these transmitters emit electromagnetic energy upon being energized by DC pulse signals. Based on the strength of the electromagnetic fields measured by the sensor, a processor determines the position and orientation of the sensor relative to the transmitter.
An electromagnetic navigation system does not require a line-of-sight path between the transmitter and sensor. A surgeon could allow his/her arm to enter the space between the system's transmitter and sensor without being concern that such action will result in the interruption of the generation of the object position and orientation-defining data.
Nevertheless, care must be taken when using an electromagnetic navigation system, especially in a surgical setting. This is because metal objects exposed to electromagnetic waves from a first source, in turn, generate their own electromagnetic waves. When ferrous metals, such as cold rolled steel, are exposed to magnetic waves, the metal itself becomes magnetized. The metal, in turn, generates its own magnetic fields. This added magnetic field is sensed by the sensor. This added magnetic field thus introduces an error into the magnetic field measurements made by the sensor.
One proposed means to reduce the adverse affects ferrous metals have on a surgical navigation system is to provide shielding around the surgical site. This shielding is positioned between the space in which the components of the surgical navigation system are located and the space into which the metal objects may be introduced. The shield blocks the transmission of the electromagnetic waves. Thus, metal objects that are separated by the shield neither become magnetized nor generate their own magnetic fields that can affect the measurements made by the navigation system sensor.
One proposed shielded magnetic field surgical navigation system includes an operating room table to which a metal plate formed from material with a high magnetic permeability is mounted. The system transmitter is mounted to the table at a fixed relationship to the metal plate. The plate blocks the adverse affects that would otherwise be introduced by the presence of ferromagnetic objects located on the side of the plate opposite the shield.
In the above system, the highly magnetic permeable plate itself is a source of interference. However, because the transmitter and plate are at fixed relationship relative to their relative position and orientation, this interference can be calculated at system assembly. The processor used to convert the sensor data to data that accurately indicates position, based on the known interference data, factors out the affect of this interference.
A clear limitation of the above-described system is that it requires the transmitter to remain at a fixed location relative to the shielding plate. This means the shielding could not be used with a system with a moving transmitter. An advantage of providing a moving transmitter is that it allows the transmitter to be placed relatively close to the sensor the position of which is being tracked. This relatively close positioning makes its possible to transmit relatively low power signals to the sensor will still ensuring the sensor is able to generate signals that, with a high degree of accuracy, can be used to determine its position and orientation.
Moreover, the proposed system is understood to require the placement of the transmitter relatively close to the shield, 10 cm or less. This is believed to require the retrofitting of existing operating room beds (tables) to facilitate the close placement of the transmitter and shield or the instillation of an operating room table especially designed to accommodate these components.